Piracy

Key resellers are really, truly awful. In many cases the keys are purchased from legitimate sites using stolen credit card numbers. The key resellers plead ignorance as to where the keys come from, but it's an open secret at this point. If you don't want to pay the Steam/Gog price, piracy is less awful because you won't be fueling a criminal enterprise and there's no chance your Steam/Gog account will get a stolen key revoked.

Credit card fraud and software keys actually ends up being paid for by the rest of us. Fraudulent transactions and chargebacks lead to higher merchant fees, and those costs end up getting passed on to legitimate purchasers.